The present invention relates to wave energy converters and, in certain aspects, methods and apparatus for harnessing hydrokinetic renewable energy from ocean waves to produce electricity.
Wave energy is a concentrated form of renewable energy generated by friction between the water surface and the wind. The energy is built up by the wind on the open seas and then transported to locations closer to the shore, where it can be extracted with wave energy converters. Due to the high energy density of ocean waves, wave power is very area efficient and the average energy content changes more slowly and predictably compared to the wind etc. The resources are vast and can be harvested close to populated areas.
However, there are challenges that must be solved before wave power can be commercially viable. Intermittent and highly fluctuating energy from the ocean waves must efficiently be converted into a steady output of electricity that is suitable for the power grid. Ocean waves vary in height, length, direction and time period (velocity) from wave to wave at a given sea state. A sea state is defined by the significant wave height (Hs) which is calculated from the average of the highest ⅓ of 100 waves in a row. The sea state will change slowly but largely over time; in storm conditions the average energy content in a sea state can be over 100 times higher than during normal conditions (annual average).
A majority of patents directed to wave energy converters (WECs) are directed to reciprocating WECs. Reciprocating WECs use wave-driven reciprocating motions such as heave, pitch, sway, reciprocating bending or curving, etc. for energy harvesting. Well-known examples that have been developed or under development include PowerBuoy (WO2007130334 A3), Pelamis (WO2000017519 A1) and Oyster (WO2006100436 A1) among others. This class of WECs relies on resonance to achieve a desired efficiency. However, in ocean environments the wave frequency spectrum is complicated, and the dominant frequency component changes day by day. That poses a tremendous challenge to the WEC design: a WEC with a fixed resonant frequency promises a relatively simple (thus robust and low-cost) structure, but it responds well to a very narrow band of the wave frequency only; whereas a WEC with a tunable resonant frequency leads to a much sophisticated (thus vulnerable and high-cost) structure, although the WEC's wave-frequency response range can be broadened to a certain extent.
In contrast to the reciprocating WEC class, there is a unidirectional WEC class. Unidirectional WECs perform unidirectional rotation that is directly driven by waves. Therefore, they do not use the resonance principle to improve the efficiency and, thus, have no need for frequency tuning. This frequency tolerance promises simple WEC designs to work efficiently in a very broad wave-frequency range. The unidirectional WEC class can be further divided into a horizontal axis group and a vertical axis group.
A horizontal axis unidirectional WEC features a rotary axis/shaft that is horizontally oriented. Examples of this type include a Savonius WEC (Faizal et al, 2009, Renewable Energy, 35, 164-169), a variation from the Savonius WEC (U.S. Pat. No. 8,206,113 B2), and a cycloidal WEC (U.S. Pat. No. 8,100,650 B2). Generally, a horizontal axis WEC prefers orthogonal alignment of the shaft with respect to the wave propagation direction for the best efficiency. In other words, the WEC shaft needs to be along the wave crest direction. When the wave direction changes, the WEC is expected to realign itself accordingly. However, realizing such realignment through a natural passive control (by means of hydrodynamic design) is difficult for this type of WECs, but a forced active control drastically complicates the WEC design.
The realignment becomes needless for vertical axis unidirectional WECs, which have their rotary shafts vertically oriented. For a successful vertical axis unidirectional WEC design, waves in any propagation directions work the same way in driving the WEC for a unidirectional rotary motion. One example of this type is Wave Rotor (WO2010011133 A1). It rotates well in simple waves but stops in irregular waves, according to its developer. Another example is published in patent application (WO2012166063 A1). In the only drawing of this application, the WEC—a vertical axis water turbine—was simply illustrated by a rectangle. In the description nothing on the structure or working principle of this turbine, which was supposed to be the core technology, has ever been provided at all.
Overall and in principle, the vertical axis unidirectional WECs have no need for either frequency tuning or realignment to cope with constantly changing wave conditions for high efficiency. Therefore, this WEC group has a great potential to result in very simple WEC designs that do not rely on any information or intervention from weather forecasting, sensing, electrical and/or mechanical control, etc. In harsh ocean environments, simplicity leads to robustness and guarantees low capital, operation and maintenance costs. There remains a need for additional methods and apparatus for vertical axis unidirectional WECs.